This invention relates to a process and apparatus for air separation by rectification, including argon rectification.
A process, in which raw argon is recovered following an air separation, is known from DE-OS 34 36 897. Air is compressed, prepurified, cooled and preseparated in the high pressure stage of a two-stage rectification into a nitrogen-rich fraction and an oxygen-rich liquid and both fractions are fed at least partially to the low-pressure stage of the rectification and separated into oxygen and nitrogen. An argon-containing oxygen stream is removed from the low-pressure stage and conveyed to a raw argon rectification stage. In this process, the raw argon rectification is performed at the pressure at which the argon-containing oxygen fraction is removed from the low-pressure stage, which is typical for this type of system. Liquid oxygen is then fed back from the raw argon rectification to about the same location of the low-pressure stage where the argon-containing oxygen fraction is withdrawn.
Such a process is advantageous if the low-pressure stage and the raw argon rectification are both performed substantially at atmospheric pressure. But in many cases, the oxygen and/or nitrogen that are produced in the low-pressure stage are needed at superatmospheric pressures, for example, in coal gasification plants or for the injection of nitrogen in the extraction of crude oil or natural gas. In such cases, for the production of compressed nitrogen and/or compressed oxygen it is economically more advantageous to operate the low-pressure stage at an increased pressure, about 2.0 to 8.0 bars, than to subsequently compress the products recovered at substantially atmospheric pressure. In the known processes, the raw argon rectification must also be performed under such increased pressures; however, under increased pressures lower argon yields are obtained from the crude argon rectification.